1. Field of the Invention
The invention relates to dual-pass continuously variable transmission systems, that is, systems having two drive paths. In the system, the transmission ratio and belt tension are controlled by a hydraulic circuit that provides a primary, or ratio determining, pressure to regulate the drive pulley and a secondary, or belt tensioning, pressure to regulate the driven pulley. More particularly, the invention relates to a control system for regulating the primary, secondary, and line pressures of the hydraulic control circuit, including means for alternating the supplies of primary and secondary pressures between the first and second pulleys of the transmission system in response to change in the drive path so that the drive pulley always receives the primary, ratio determining, pressure and the driven pulley always receives the secondary, belt tensioning, pressure.
2. Description of the Prior Art
Continuously variable transmissions (CVTs) are known in the art. The typical CVT employs an infinitely variable speed mechanism comprised of a pair of adjustable pulleys, each pulley having at least one sheave that is axially fixed and another sheave that is axially movable relative to the first. A flexible belt of metal or elastomeric material intercouples the pulleys. The inner faces of the sheaves of the pulley are bevelled or chamfered so that as the axially displaceable sheave moves relative to the fixed sheave, the distance between the sheaves and, thus the effective pulley diameter may be adjusted.
The displaceable sheave includes a fluid constraining chamber for receiving fluid to move the sheave and thus change the effective pulley diameter. As fluid is added to or exhausted from the chamber, the effective pulley diameter changes. Generally the effective diameter of one pulley is moved in one direction as the effective diameter of the other pulley is moved in the opposite direction. This enables adjustment of the ration between the effective diameters of the pulleys.
A dual-pass continuously variable transmission (CVT) is a CVT which provides two alternate power paths between the input and output shafts. One of the paths utilizes the first of the adjustable pulley of the infinitely variable speed mechanism as the driving pulley and the second path utilizes the second of the infinitely variable pulleys as the driving pulley. The first and second variable pulleys are disposed on first and second intermediate shafts, respectively. When the first drive path is utilized, the second variable pulley and second intermediate shaft are driven by the first pulley of the infinitely variable speed mechanism. When the second drive path is utilized, the first pulley and first intermediate shaft are driven by the second pulley of the infinitely variable speed mechanism.
The input shaft is connected to the first intermediate shaft by a reduction gear and a one-way clutch that is engaged when the first drive path is utilized. The input shaft is connected to the second intermediate shaft by an overdrive gear and an engagable jaw clutch. The jaw clutch is released when the first drive path is utilized to allow the overdrive gear to rotate independently of the second intermediate shaft, which may be rotating at a slower rate. The output shaft is connected to the second intermediate shaft by an overdrive gear and one-way clutch that is engaged when the first drive path is utilized. The output shaft is connected to the first intermediate shaft through a reduction gear and an engagable jaw clutch. The jaw clutch is released when the first path is utilized and engaged when the second drive path is utilized.
To increase the output speed, the CVT initially establishes the first drive path with both one-way clutches engaged to effect a first pass through the CVT. The infinitely variable speed mechanism then changes continually from underdrive to overdrive thereby increasing the rotational speed of the second intermediate shaft and the output shaft. The second intermediate shaft will eventually be rotating at the same speed as the overdrive gear intermeshed with the teeth of a gear rotating with the input shaft. When this occurs, both jaw clutches can be engaged and the second drive path is established for a second pass through the CVT. The two one-way clutches overrun to allow the second pulley to drive the first pulley. The output shaft thus undergoes a second increase in output speed during the second pass as the ratio between the second pulley and the first pulley is increased from underdrive to overdrive. Such a system is disclosed in U.S. patent application No. 564,856 filed Dec. 23, 1983, now abandoned, which is incorporated herein by reference.
To increase the ratio across the variable component in the drive path that is currently established, the movable sheave of the driving pulley must be moved inward and the belt on the driving pulley forced outward. This requires an appropriate hydraulic pressure to be supplied to the constraining chamber of the driving pulley. In order to assure that a smooth transfer is achieved when the drive path alters on transition from the first pass to the second pass, or vice versa, it is important that the hydraulic pressure does not change prematurely as this is likely to cause the ratio across the variable component to change away from the ratio at which engagement of the jaw clutches is possible. This will create a mismatch between the speed of rotation of the two halves of the engaging clutch which will cause excessive wear to the clutches, particularly if they are jaw clutches.
Jaw clutches are generally utilized in preference to friction clutches to change the drive path of a dual-pass CVT as the torques involved in the engagement of a clutch between a gear and one of the intermediate shafts may be as high as 300-400 ft. lbs. A large area and high pressure would therefore be required for a friction clutch to engage at such torques. Also the time involved in engaging a friction clutch at such torques would be far greater than that for a jaw clutch which will engage within hundredths of a second and may only be 3.5 inches in diameter.
The use of a pair of jaw clutches, however, presents additional problems. Jaw clutches cannot engage when there is a significant difference in speed between the two clutch halves or excessive clutch wear will result. In addition, as each jaw clutch has a finite number of teeth, the two jaw clutches do not necessarily engage simultaneously. Therefore, there is likely to be some delay between the engagement of the two clutches. This delay is dependent upon the number of teeth in each clutch and the small speed difference across the clutch members. The delay may be on the order of hundredths of a second.
It is therefore important that the hydraulic pressures determining the ratio across the variable component and the belt tension do not change until both jaw clutches are engaged or the clutch halves may move away from synchronicity and the smooth transfer from one pass to another will be disrupted. In a hydraulically controlled dual-pass CVT it is therefore important to have a system that provides the desired primary and secondary hydraulic pressures to the driver and driven pulleys, respectively, for given input conditions in each of the two passes.